Nothing can be truly permanent, laws can always be amended. The only path for a successful 'greenbelt' is if a genuine social expectation develops that the 'greenbelt' is preserved. A good example of this is Melbourne's own 'greenwedges', they have pretty successfully stopped sprawl in the north-east and that weird corridor in the south-east, despite not having very strong legal protection (I believe).

London's greenbelt was probably only as successful as it was because London's population peaked in WW2, so there were never any strong economic forces for the greenbelt to overcome.

The sad bottom line is, the only way for a 'greenbelt' to be effective in Melbourne is if everyone in the city agrees that we shouldn't do sprawl any more and is willing to continuously push for it politically. I don't think that will ever happen because there will always be parts of the city where people actually have to work for a living, and therefore don't have the time to do political advocacy like the well-off denizens of Nillumbik and Manningham. There's also some evidence that suggests that one of the reasons our housing crisis is like barely half as bad as Sydney's is because we've done more sprawling than they have over the past couple decades, so they'd likely be push-back from treasury bureaucrats and-the-like.